The examination of entanglement across various degrees of freedom has been pivotal in augmenting our understanding of fundamental physics, extending to high dimensional quantum states, and promising the scalability of quantum technologies. In this paper, we demonstrate the photon number path entanglement in the frequency domain by implementing a frequency beam splitter that converts the single-photon frequency to another with 50% probability using Bragg scattering four-wave mixing. The two-photon NOON state in a single-mode fiber is generated in the frequency domain, manifesting the two-photon interference with two-fold enhanced resolution compared to that of single-photon interference, showing the outstanding stability of the interferometer. This successful translation of quantum states in the frequency domain will pave the way toward the discovery of fascinating quantum phenomena and scalable quantum information processing.

对不同自由度纠缠的研究对于增强我们对基础物理的理解、扩展到高维量子态以及保证量子技术的可扩展性至关重要。在本文中，我们通过实现频率分束器来演示频域中的光子数路径纠缠，该频率分束器使用布拉格散射四波混频以 50% 的概率将单光子频率转换为另一个频率。单模光纤中的双光子NOON态是在频域产生的，体现出双光子干涉的分辨率是单光子干涉的两倍，显示出干涉仪出色的稳定性。频域中量子态的成功转换将为发现令人着迷的量子现象和可扩展的量子信息处理铺平道路。